The components of the insulin receptor have been examined in rat liver membranes and IM-9 lymphocytes using SDS polyacrylamide gel electrophoresis (PAGE) in conjunction with specific immunoprecipitation and radiation inactivation on SDS gels of surface labeled cells. The subunit structure of the insulin receptor of IM-9 membranes appears complex, containing 4 subunits; the largest of which has a mol. wt. of 80-100,000. This corresponds relatively closely to the size of the insulin binding site in rat liver membranes as determined by radiation inactivation. In addition, in the intact membrane there appears to be a high mol. wt. affinity regulatory protein.